Fe(Ⅲ) Replacement/Photolysis Precipitation For Treatment Of Cu(Ⅱ)-complexed Wastewater

The rapid development of electroplating and printed circuit board industry inevitably accompanies a large amount of heavy metal wastewaters. A wide variety of complexing agents and additives, such as EDTA, NTA and citric acid, were used in the above industry, resulting in the formation of heavy metal complexes in effluent. Traditional processes for heavy metal removal, such as chemical precipitation, adsorption and micro-electrolysis, could not effectively remove metal complexes in a cost effective manner.Herein we proposed a new method for treatment of wastewater containing heavy metal complexes with carboxyl group, i.e., Fe(Ⅲ) replacement followed by photolysis precipitation. In theory, the added Fe(Ⅲ) could replace and release target heavy metals from the complexes, and the newly formed Fe(Ⅲ) complex could be photodegraded under UV irradiation. Finally, both metals were removed by conventional precipitation method. To test the efficiency of the proposed process, Cu-EDTA was employed as a model metal complex. The main experiments and results are summarized as follows:(1) For treatment of a synthetic Cu-EDTA solution, some key parameters of the process, i.e., the dosage of iron, initial pH values, pH values for precipitation, the illumination time and TOC removal were optimized. For Cu-EDTA at0.3mM, the optimal conditions were:the molar Fe/Cu ratio,≥8; initial pH,1.5-3.0; pH for precipitation,9-11; the illumination time,≥8min, where Cu in solution could be reduced to below0.5mg/L, and the TOC removal rate was near40%. The commonly occurring ions posed no influence on Cu(Ⅱ) removal. The obtained precipitate was magnetic, containing Fe3O4and other metal oxides.(2) To evaluate the efficiency of the combination process for treatment of industrial effluent, an electroplating wastewater was sampled from Huizhou Industrial Park with pH=2, Cu=125mg/L, and TOC=37mg/L. When the molar Fe/Cu ratio≥5, the precipitation pH was set as10, and the illumination time was longer than20min, Cu(Ⅱ) in the effluent could be removed to below1mg/L, while TOC removal rate was about10%. Sodium hydroxide is more efficient than calcium hydroxide for the final precipitation.(3) The proposed mechanism of Fe(Ⅲ) replacement-photolysis precipitation possibly involved two aspects:Fe(Ⅲ) replacement occurred because the complexing ability of Fe(Ⅲ) with EDTA was much stronger than Cu, and photolysis process could be realized based on the electron transfer from ligand to metal (LMCT) under UV irradiation.